Protein kinase C (PKC) functions in processes relevant to carcinogenesis, tumor cell metastasis, and apoptosis. Safingol, an optical isomer (the L-threo enantiomer) of dihydrosphingosine, is a specific inhibitor of PKC and may represent a novel target for anti-cancer therapy. Preclinical animal studies show that safingol alone has minimal effects on tumor cell growth, but combination of this compound with conventional chemotherapy agents dramatically potentiates their anti-tumor effects. It has been suggested that many chemotherapeutic agents exert their anti-tumor effects by inducing apoptosis.
A large body of evidence indicates a fundamental role for the involvement of protein kinase C (PKC), family members of serine/threonine protein kinases, in processes relevant to neoplastic transformation, carcinogenesis, and tumor cell invasion of surrounding tissues (1–3). Consequently, PKC may represent a novel target for anti-cancer therapy. Safingol, the L-threo enantiomer of dihydrosphingosine, is a specific inhibitor of PKC (4). Preclinical animal studies show that safingol is non-toxic at doses that achieve serum levels sufficient to inhibit PKC enzyme activity (5). While safingol has negligible impact on tumor cell growth in vivo, the combination of safingol with conventional chemotherapeutic agents such as doxorubicin and cisplatin significantly potentiates the anti-tumor effects of these drugs (6).
Based on these observations, safingol, used in combination with doxorubicin, has become the first PKC specific inhibitor to enter clinical trials. The mechanism by which safingol potentiates the activity of chemotherapeutic agents is unclear, although inhibition of P-glycoprotein phosphorylation and reversal of the multidrug resistant (mdr) phenotype have been suggested (7,8). While this hypothesis can explain the synergism achieved with combinations of safingol and doxorubicin, it does not explain the synergism reported for combinations of safingol with drugs that are not believed to produce resistance by the mdr mechanism (e.g., cisplatin) (6), nor does it explain safingol-induced effects that occur in tumor cell lines that do not express the P-glycoprotein (8). Therefore, pathways other than P-glycoprotein inhibition are likely to be involved in the safingol-mediated enhancement of chemotherapy.
It has been suggested that the anti-tumor activity of many chemotherapeutic agents (e.g., cisplatin and etoposide) is a consequence of their induction of apoptosis (9). In this context it has been proposed that activation of PKC acts as an antagonist to apoptosis, whereas inhibition of PKC promotes apoptosis (10–12). Thus, safingol-mediated potentiation of chemotherapy might be attributed to its PKC inhibitory effect, subsequently leading to increased apoptosis after drug-induced damage.
The present studies sought to determine the extent to which whether safingol by itself, or in combination with a specific chemotherapeutic drug (e.g. mitomycin-C, MMC), would promote apoptosis in gastric cancer cells. Furthermore, applicants investigated whether the p53 status of these cells influences the development of apoptosis after treatment with safingol and MMC.